Toast the white sesame seeds in a dry skillet over medium heat, stirring, until lightly golden, about 5 minutes. Remove, then toast the black sesame seeds in the skillet; remove and set aside.
Toss the salmon, onion, soy sauce, chili-garlic sauce, white sesame seeds, sesame oil and ginger in a medium bowl. Cover and refrigerate until cold, about 30 minutes.
Halve and pit the avocados, then carefully remove the peel with your fingers. Cut shallow cross-hatch marks into the center of each avocado half (this will make it easier to eat). Squeeze the lemon over the avocados and season with salt. Sprinkle the cut sides of the avocados with the black sesame seeds, gently pressing to adhere. Fill the centers of the avocados with the salmon poke and top with sliced radishes and scallions. Serve immediately.